Imagine a basketball player who just took a shot and scored. This player is very likely to repeat the same movement because it was rewarded with points for the team. It is known that when one does a rewarding action - like the one described above - one tends to repeat that same behavior. However, we do not know what is happening at the cellular level to bias the system into repeating a rewarded action over a non-rewarded one. We believe this biasing mechanism is happening at the sub cellular level, particularly at the dendritic spines. Specifically, by changing the structure of the spines and hence the strength of the synapses, one increases the probability of that particular cell firing causing the bias for repeating the rewarded action. My main interest is to understand the mechanism by which this bias occurs.